1. Field of the Invention
The present invention relates to a titanium alloy part such as a titanium alloy spring, and a method for producing the same.
2. Description of the Related Art
As compared to iron, titanium excels in physical properties which are important to any structural or functional part (or member) of a mechanical apparatus. Specifically, titanium has a lower density than that of iron, and has high strengths (e.g., tensile strength) relative to its specific gravity. Moreover, titanium has a Young's modulus which is about half of that of iron, and thus shows excellent elastic characteristics. Therefore, a structural or functional part which has a light weight, a high strength, and a good elasticity can be formed from titanium. A titanium alloy which is composed by adding various elements to titanium can have further improved characteristics.
In spite of such advantages, structural or functional parts composed of titanium or titanium alloys have only been used for specific applications such as aircraft or golf club shafts. The reason is that, conventionally, titanium and titanium alloys can only be produced at a higher cost than that of iron.
In recent years, however, methods for producing titanium alloys at lower costs have been developed, so that cost-related constraints on using titanium alloys as structural or functional parts are being relaxed. Therefore, studies have been directed to using titanium alloys in products in various fields by taking advantage of the aforementioned superior characteristics of titanium.
In particular, when a spring is composed of a titanium alloy (hereinafter, such a spring will be referred to as a “titanium alloy spring”), the weight per unit length of wire material composing the spring can be reduced due to the low density of titanium. The small Young's modulus makes it possible to reduce the number of turns made in the spring, and reduce the spring height and the total length of the wire material for the spring which are necessary for obtaining a given amount of contraction and expansion. Therefore, a titanium alloy spring can have a weight which is reduced by about 60% from that of a steel spring which has similar levels of functionalities. By using such light-weight springs for suspensions of a vehicle, the total weight of the vehicle can be reduced, and vibrations can be dampened quickly, whereby the vehicle running properties can be enhanced.
Conventionally, when producing a steel spring, objects (called “shot medium”) such as cut wires of steel or cast steel balls are shot against the surface of the spring to cause plastic deformation of the surface, thus creating a compressive stress in the interior of the spring near the surface, whereby the durability of the spring is improved. This treatment is called “shot peening”. In the case where a compressive stress has been created near the surface of the spring, even if a flaw is formed in the surface, the compressive stress will act in a direction which does not allow the flaw to expand. As a result, the flaw is prevented from expanding and causing destruction of the spring.
Also when producing a spring composed of a titanium alloy, shot peening is known to realize an improved durability, as is disclosed in Japanese Laid-Open Patent Publication No. 5-195175 and Japanese Laid-Open Patent Publication No. 5-112857.
However, a study conducted by the inventors of the present invention has shown that the shot peening conditions which are disclosed in the aforementioned publications do not actually guarantee that a spring having a sufficient durability, especially a sufficient fatigue strength, will be obtained.